Scintigraphy used in nuclear medicine examinations is one type of nuclear medicine imaging techniques. In this technique, a radiopharmaceutical agent is administered into a subject's body, and gamma rays, which are emitted by decay of nuclides in the radiopharmaceutical agent, are captured by a single-photon emission computed tomography (SPECT) apparatus or other means and imaged. In scintigraphy, pixel values are normalized, that is, pixel values are converted to values comparable with those of other images, for the purposes of follow-up examinations, comparison among images, and others. Normalization methods include a manual method in which an operator visually performs level adjustment, and an automatic method in which normalization is performed automatically.
The manual method is advantageous in that it does not need any specific program; however, it is disadvantageous in that the results depend on the level of operator's techniques. For example, in the method of visually performing level adjustment, the results are subject to senses of an operator, which is likely to cause variations in the results. In another method, a region of interest (ROI) is defined on the thoracic spine and its maximum value is used for normalization. This method, however, is not usable when abnormal accumulation is present in the defined ROI.
Patent Literature 1 and Non Patent Literature 1 described below disclose techniques of semi-automatically determining a reference value for normalizing a bone scintigraphy image, which is a field of application of scintigraphy. These pieces of literature describe analyzing a histogram of a bone scintigraphy image using a multi-threshold method disclosed in the following Non Patent Literature 2, determining a boundary value between pixel values corresponding to abnormal accumulation of a radiopharmaceutical agent and pixel values corresponding to normal accumulation thereof, and normalizing the bone scintigraphy image using the boundary value as a reference.